This is a nationalization of PCT/JP00/04601 filed Jul. 10, 2000 and published in Japanese.
The present invention relates to a strain gauge to be used, for example, in a pressure sensor or a load cell.
Conventionally, a thin film strain gauge has been used in an automotive pressure sensor, a pressure sensor for measuring pressure, or load, a load cell and the like.
That is, a thin film strain gauge film is formed on an insulating film directly provided on the metal diaphragm of a pressure sensor or the beam member of a load cell and is used for pressure measurement, etc.
A case in which a strain gauge is used in a pressure sensor will be described with reference to FIGS. 3 and 4.
FIG. 3 is a schematic diagram illustrating a main portion of a pressure sensor, and FIG. 4 is a circuit diagram of the pressure sensor shown in FIG. 3.
As shown in FIG. 3, when using a strain gauge in a pressure sensor, a gauge pattern consisting of four gauge films (12, 13, 14, and 15) is formed, for example, on a metal diaphragm 5 through the intermediation of an insulating film by photolithography process or the like.
And, FIG. 4 is a circuit diagram of the pattern.
Due to the above-described construction, any change in the pressure (stress) on the diaphragm 5 results in the diaphragm 5 being deformed accordingly.
Thus, deformation of the diaphragm 5 results in a change in the strain,amounts of the respective gauge films 12, 13, 14, and 15 formed on the diaphragm 5.
As shown in FIG. 3, the stress coefficient on the diaphragm 5 varies according to the position, so that the strain amounts of the respective gauge films 12, 13, 14, and 15 are not the same. When the strain amounts of the respective gauge films 12, 13, 14, and 15 are thus changed in response to a change in pressure, the electrical resistances of the respective gauge films 12, 13, 14, and 15 are also changed.
Thus, the resistance values R1, R2, R3, and R4 shown in FIG. 4 change in accordance with the pressure, so that the output voltage V1 with respect to the input voltage V0 changes. Since the output voltage V1 changes linearly according to the pressure change, it is possible to measure the pressure from the output voltage V1.
However, the above-described prior art technique involves the following problems.
The main characteristics required of the material for the gauge film in achieving an improvement in the performance and characteristics of such a pressure sensor or the like are as follows: (1) small temperature coefficient of resistance (hereinafter referred to as TCR), (2) high gauge factor, (3) high resistivity, (4) small change in resistance with time, (5) independence of the gauge factor of temperature, etc.
In view of this, a metal strain gauge, for example, of an NiCr type metal, has been put into practical use. However, as a result of the improvement in the various performances of vehicles and other apparatuses in which the pressure sensor or the like is used, there increases a demand for an improvement in the performance and characteristics of various parts and sensors, and it is difficult for the conventionally used materials to meet such a demand.
For example, when used as the gauge material, silicon semiconductor, which has a high gauge factor of approximately several tens, exhibits high sensitivity with respect to pressure change, thus proving an effective material. However, its TCR is rather large, and its linearity is deficient, so that, for precise measurement, it is necessary to provide a special device for temperature compensation.
When used as the gauge material, a metal material consisting, for example, of an NiCr type alloy or a CuNi type alloy, has a small TCR (several to several hundred ppm/K). On the other hand, it has a rather low gauge factor of approximately 2, which leads to low sensitivity to pressure change.
As a gauge material having a high gauge factor, a Cr type material (with a gauge factor of approximately 10) is being studied. However, due to its poor output temperature characteristics (TCR, etc.), it has not been put into practical use yet.
In this way, selection of a material with a small TCR involves a low gauge factor. On the other hand, selection of a material with a high gauge factor involves a large TCR. Thus, conventionally, it has been impossible to simultaneously achieve a high gauge factor and a small TCR.
From this point of view, there has been also proposed a metal thin film strain gauge in which a resistor consisting of an Fexe2x80x94Crxe2x80x94Al alloy thin film is formed into a film by evaporation, sputtering or the like as the resistor of a metal thin film strain gauge (See, for example, Japanese Patent Application Laid-open No. Hei 6-137804).
However, even in this strain gauge, the gauge factor is still rather low. Thus, there has been a demand for a strain gauge having a high gauge factor and a decreased temperature coefficient of resistance.
The present invention has been made with a view toward solving the above problem in the conventional art. It is an object of the present invention to provide a strain gauge having a high gauge factor and a decreased temperature coefficient of resistance (TCR) to thereby achieve an improvement in terms of performance.
To achieve the above object, in accordance with the present invention, a strain gauge is characterized by comprising a laminated structure that includes a first layer formed of a positive TCR material and a second layer formed of a negative TCR material.
Thus, even if, to achieve an increase in the gauge factor of the strain gauge as a whole, materials having a high gauge factor are selected as the materials for forming the first and second layers, it is possible to reduce the temperature coefficient of resistance of the strain gauge as a whole since the materials have positive and negative temperature coefficients of resistance, respectively. Further, due to the laminated structure, each layer provides an interface effect to thereby further increase the gauge factor. The first layer is preferably formed of a crystalline metal material, and the second layer is preferably formed of an amorphous metal. The second layer is preferably formed of tantalum.